The invention relates to any GPS receiver, however, the primary application would be a centimeter accurate global positioning system (GPS) receiver for real-time kinematic (RTK) measurement. The satellite positioning system (SATPS) can include different satellite systems or combinations of the satellite systems. One of those systems is a global positioning system (GPS).
The GPS is a system of satellite signal transmitters, with receivers located on the Earth's surface or adjacent to the Earth's surface, that transmit information from which an observer's present location and/or the time of observation can be determined. There is also the Global Navigational System (GLONASS), which can operate as an alternative GPS system.
The GPS is part of a satellite-based navigation system developed by the United States Defense Department under its NAVSTAR satellite program. A fully operational GPS includes up to 24 Earth orbiting satellites approximately uniformly dispersed around six circular orbits with four satellites each, the orbits being inclined at an angle of 55.degree. relative to the equator and being separated from each other by multiples of 60.degree. longitude. The orbits have radii of 26,560 kilometers and are approximately circular. The orbits are non-geosynchronous, with 0.5 sidereal day (11.967 hours) orbital time intervals, so that the satellites move with time relative to the Earth below. Theoretically, three or more GPS satellites will be visible from most points on the Earth's surface, and visual access to three or more such satellites can be used to determine an observer's position anywhere on the Earth's surface, 24 hours per day. Each satellite carries a cesium or rubidium atomic clock to provide timing information for the signals transmitted by the satellites. Internal clock correction is provided for each satellite clock.
Each GPS satellite continuously transmits two spread spectrum, L-band carrier signals: an L1 signal having a frequency f1=1575.42 MHz (nineteen centimeter carrier wavelength) and an L2 signal having a frequency f2=1227.6 MHz (twenty-four centimeter carrier wavelength). These two frequencies are integral multiplies f1=1,540 f0 and f2=1,200 f0 of a base frequency f0=1.023 MHz. The L1 signal from each satellite is binary phase shift key (BPSK) modulated by two pseudo-random noise (PRN) codes in phase quadrature, designated as the C/A-code and P-code (or Y-code). The L2 signal from each satellite is BPSK modulated by only the P-code (or Y-code). The nature of these PRN codes is described below.
Use of the PRN codes allows use of a plurality of GPS satellite signals for determining an observer's position and for providing the navigation information. A signal transmitted by a particular GPS satellite is selected by generating and matching, or correlating, the PRN code for that particular satellite. Some of the PRN codes are known and are generated or stored in GPS satellite signal receivers carried by ground observers. Some of the PRN codes are unknown.
A first known PRN code for each GPS satellite, sometimes referred to as a precision code or P-code, is a relatively long, fine-grained code having an associated clock or chip rate of 10 f0=10.23 MHz. A second known PRN code for each GPS satellite, sometimes referred to as a clear/acquisition code or C/A-code, is intended to facilitate rapid satellite signal acquisition and hand-over to the P-code and is a relatively short, coarser-grained code having a clock or chip rate of f0=1.023 MHz. The C/A -code for any GPS satellite has a length of 1023 chips or time increments before this code repeats. The full P-code has a length of 259 days, with each satellite transmitting a unique portion of the full P-code. The portion of P-code used for a given GPS satellite has a length of precisely one week (7.000 days) before this code portion repeats.
Accepted methods for generating the C/A-code and P-code are set forth in the document GPS Interface Control Document ICD-GPS-200: GPS Interface Control Document, ARINC Research, 1998, GPS Joint Program Office, which is incorporated by reference herein.
A first unknown PRN code is Y-code. To prevent jamming signals from being accepted as actual satellite signals, the satellites are provided with a secret Y-code, which replaces the known P code when the "anti-spoofing" (AS) is ON. When the AS is OFF, the Y-code is turned OFF, and the known P-code (see above cited ICD-GPS-200) is used. Thus, the secret Y-code can be turned ON or OFF at will by the U. S. Government. The AS feature allows the GPS system to be used for the military or other classified United Government projects.
The C/A code is transmitted on L1 under all conditions as it is generally required to provide timing access to L1 and L2 P(Y) code. When AS is OFF, the known P code is transmitted on both L1 and L2, allowing authorized and unauthorized users alike access to full coded receiver operation on both L1 and L2 frequencies. As it is indicated above, when AS is ON, the known P code is replaced with a secret Y code on both L1 and L2.
The GPS satellite bit stream includes navigational information on the ephemeris of the transmitting GPS satellite (which includes a complete information about the transmitting satellite within next several hours of transmission) and an almanac for all GPS satellites (which includes a less detailed information about all other satellites). The satellite information transmitted by the transmitting GPS has the parameters providing corrections for ionospheric signal propagation delays suitable for single frequency receivers and for an offset time between satellite clock time and true GPS time. The navigational information is transmitted at a rate of 50 Baud. A useful discussion of the GPS and techniques for obtaining position information from the satellite signals is found in The NAVSTAR Global Positioning System, Tom Logsdon, Van Nostrand Reinhold, New York, 1992, pp. 17-90.
A second alternative configuration for global positioning is the Global Navigation Satellite System (GLONASS), placed in orbit by the former Soviet Union and now maintained by the Russian Republic. GLONASS also uses 24 satellites, distributed approximately uniformly in three orbital planes of eight satellites each. Each orbital plane has a nominal inclination of 64.8.degree. relative to the equator, and the three orbital planes are separated from each other by multiples of 120.degree. longitude. The GLONASS circular orbits have smaller radii, about 25,510 kilometers, and a satellite period of revolution of 8/17 of a sidereal day (11.26 hours). A GLONASS satellite and a GPS satellite will thus complete 17 and 16 revolutions, respectively, around the Earth every 8 days. The GLONASS system uses two carrier signals L1 and L2 with frequencies of f1=(1.602+9k/16) GHz and f2=(1.246+7k/16) GHz, where k (=1,2, . . . 24) is the channel or satellite number. These frequencies lie in two bands at 1.597-1.617 GHz (L1) and 1,240-1,260 GHz (L2). The L1 code is modulated by a C/A- code (chip rate=0.511 MHz) and by a P-code (chip rate=5.11 MHz). The L2 code is presently modulated only by the P-code. The GLONASS satellites also transmit navigational data at a rate of 50 Baud. Because the channel frequencies are distinguishable from each other, the P-code is the same, and the C/A-code is the same, for each satellite. The methods for receiving and analyzing the GLONASS signals are similar to the methods used for the GPS signals.
Reference to a Satellite Positioning System or SPS herein refers to a Global Positioning System, to a Global Orbiting Navigation System, to any other compatible satellite-based system, or combination of satellite systems that can provide information by which an observer's position and the time of observation can be determined, all of which meet the requirements of the present invention.
A Satellite Positioning System (SATPS), such as the Global Positioning System (GPS), the Global Navigation Satellite System (GLONASS), or the combination of GPS and GLONASS systems uses transmission of coded radio signals, with the structure described above, from a plurality of Earth-orbiting satellites. An SATPS antenna receives SATPS signals from a plurality (preferably four or more) of SATPS satellites and passes these signals to an SATPS signal receiver/processor, which (1) identifies the SATPS satellite source for each SATPS signal, (2) determines the time at which each identified SATPS signal arrives at the antenna, and (3) determines the present location of the SATPS satellites.
The range (Ri) between the location of the i-th SATPS satellite and the SATPS receiver is equal to the speed of light c times (.DELTA.ti), wherein (.DELTA.ti) is the time difference between the SATPS receiver's clock and the time indicated by the satellite when it transmitted the relevant phase. However, the SATPS receiver has an inexpensive quartz clock which is not synchronized with respect to the much more stable and precise atomic clocks carried on board the satellites. Consequently, the SATPS receiver actually estimates not the true range Ri to the satellite but only the pseudo-range (ri) to each SATPS satellite.
After the SATPS receiver determines the coordinates of the i-th SATPS satellite by picking up transmitted ephemeris constants, the SATPS receiver can obtain the solution of the set of the four equations for its unknown coordinates (x.sub.0, y.sub.0,z.sub.0) and for unknown time bias error (cb). The SATPS receiver can also obtain its heading and speed. (See The Navstar Global Positioning System, Tom Logsdon, Van Nostrand Reinhold, 1992, pp. 8-33, 44-75, 128-187).
The following discussion is focused on a GPS receiver, though the same approach can be used for a GLONASS receiver, for a GPS/GLONASS combined receiver, or any other SATPS receiver.
When originally put into operation by the United States Government, the GPS was not intended to provide a civilian user with centimeter-level position accuracies. However, centimeter-level position accuracies are now required for such civilian applications as surveying, mapping, etc.
Extremely accurate GPS receivers depend on phase measurements of the radio carriers that they receive from various orbiting GPS satellites. Less accurate GPS receivers simply develop the pseudoranges to each visible satellite based on the time codes being sent. Within the granularity of a single time code, the carrier phase can be measured and used to compute range distance as a multiple of the fundamental carrier wavelength. As was stated above, GPS signal transmissions are on two synchronous, but separate carrier frequencies L1 and L2, with wavelengths of nineteen and twenty-four centimeters, respectively. Thus, within nineteen or twenty-four centimeters, the phase of the GPS carrier signal will change 360.degree. degrees.
For the high accuracy positioning it is necessary to take into account not only the integer part of the carrier phase difference between two satellite receivers locked on the same satellite signal, but also the fractional part of the carrier phase difference.
Assuming that no cycle slip occurs, the integer part is constant, and one can measure only the fractional part of the carrier phase difference to achieve the high accuracy positioning.
However, if a cycle slip occurs, the lock point is lost. As a result, the constant integer number of the carrier phase difference between two satellite receivers locked onto the same satellite signal is also lost. If this is the case, the constant integer number of the carrier phase difference has to be re-established. In order to do that, the new lock point should be determined.
Thus, it is important to obtain the new lock point as soon as possible in order to quickly re-establish the constant integer number of the carrier phase difference between two satellite receivers locked on the same satellite signal.
In the prior art RTK GPS receiver the BPSK GPS signal has 2 distinct lock points that are separated by 180 degrees. The lock point can be identified by looking for the Preamble bit sequence that occurs every 6 seconds.
The RTK processor cannot make maximum use of the carrier phase measurement until the lock point is identified, or resolved. This means that following a cycle slip declaration (which could be caused by the branch of a tree, for example), the satellite measurements cannot be used in the RTK solution for up to 6 seconds.
Thus, in a tree bound applications, (for example, in forest surveying applications), the RTK GPS receiver is ineffective for up to 6 seconds following a single obscuration event.
What is needed is an improved RTK GPS receiver that incorporates a circuitry that allows a fast recovery of a lost lock point of a BPSK GPS signal.